1. Field of the Invention
The present invention relates to hot-dip aluminum coated steel sheets and strips (hereinafter called sheet or sheets) having very excellent corrosion resistance as well as excellent heat resistance.
Prior to the present time, hot-dip aluminum coated steel sheets, for their general excellent corrosion resistance and heat resistance, have been finding applications mainly in automobile gas exhaust systems and various types of heating equipment. However, in recent years, due to introduction of newly developed exhaust gas purifying systems and improvements of engine heat coefficient, hot-dip aluminium coated steel sheet materials, conventionally used in the automobile gas exhaust systems, have been confronted with new problems such as severe corrosion as being experienced in exhaust gas mufflers, which is caused by changes in the composition and temperature of the exhaust gas to which the materials are exposed. In efforts to overcome the problems, particularly for the purpose of enhancing the corrosion resistance of the mufflers, stainless steel sheet materials have been increasingly used for the mufflers. The stainless steel mufflers, on the other hand, have a defect in that they cannot provide satisfactory protection on the outer portion of the mufflers from the corrosion caused by calcium chloride and sodium chloride contained in snow-melting agents sprayed on highways during the winter season.
Thus, up to now, both the conventional hot-dip aluminum coated steel sheets and stainless steel sheets have been found to be unsatisfactory due to their respective in that the former does not provide enough protection to the inside of the mufflers against the corrosion by the condensate liquid of the exhaust gas, and the stainless steel mufflers are susceptible to the outside corrosion.
For improving the corrosion resistance of the hot-dip aluminum coated steel sheet materials, the conventional practice is to apply a chromate treatment after the aluminum coating, or to increase the thickness of the aluminum coating itself. In the chromate treatment, when applied to the automobile mufflers, where the materials are subjected to 300.degree. to 400.degree. C., the chromate film considerably loses the corrosion resistance, and on the other hand the increased thickness of the aluminum coating very often causes the peeling-off of the coatings during the forming of the mufflers.